Relation between oxidation microstructure and the maximum energy product loss of a Sm₂Co₁₇ magnet oxidized at 500 ℃

The oxidation microstructure and maximum energy product (BH)_max loss of a Sm(Co_0.76, Fe_0.1, Cu_0.1, Zr_0.04)₇ magnet oxidized at 500 ℃ were systematically investigated. Three different oxidation regions were formed in the oxidized magnet: a continuous external oxide scale, an internal reaction layer, and a diffusion zone. Both room-temperature and high-temperature (BH)_max losses exhibited the same parabolic increase with oxidation time. An oxygen diffusion model was proposed to simulate the dependence of (BH)_max loss on oxidation time. It is found that the external oxide scale has little effect on the (BH)_max loss, and both the internal reaction layer and diffusion zone result in the (BH)_max loss. Moreover, the diffusion zone leads to more (BH)_max loss than the internal reaction layer. The values of the oxidation rate constant k for internal reaction layer and oxygen diffusion coefficient D for diffusion zone were obtained, which are about 1.91 × 10^-10 cm²/s and 6.54 × 10^-11 cm²/s, respectively.